In the electronics industry, ever-present goals are to minimize integrated circuit chip packaging and to minimize the lengths of interconnections between chips. Multi-chip modules are designed to address these goals. Multi-chip modules package and interconnect a plurality of chips in a functional relationship. The advantages of multi-chip modules are primarily in terms of packaging density and operational speed. The main disadvantage involves the cost of manufacturing such modules.
Among the reasons for the high cost of multi-chip modules is the relatively low manufacturing yield of the modules. As the number of integrated circuit chips within a package increases, the likelihood of a defect within the package also increases. Typically, testing of the individual chips prior to interconnection with a multi-chip module is not possible. Later detection of a defective chip within a module may result in the entire module having to be discarded.
An interconnection technique which permits pretesting of chips is referred to as "tape automated bonding." This fabrication procedure utilizes a continuous insulated tape which is similar to photographic film to provide a planar substrate for chips that are attached to individual sections, or frames, of the tape. A spider-like metal pattern of conductive traces is etched on each frame. The traces may either "fanout," i.e. radiate from the center of the frame to the four edges, or may be four sets of parallel lines, with each set extending perpendicularly from one edge of a chip. The chip is carefully aligned over the center of the frame so that the contacts of the chip are precisely located at corresponding conductive traces in the central portion of the frame. The chip is then attached to the tape automated bonding frame. This connection of the chip contacts to the inner portion of the frame is referred to as "inner lead bonding."
After the inner lead bonding has been performed, the integrated circuit chip may be tested. The chip can be thoroughly exercised electrically. After testing of a chip, the outer leads of the frame are microbonded to pads on a substrate. The attachment of the conductive traces of the frame to the pads of the substrate is referred to as "outer lead bonding." The substrate is that portion of the multi-chip module which permits mounting of more than one lead frame for electrical communication between the chips supported by the frames.
Proper alignment of the lead frame with the substrate during outer lead bonding is critical. The conductive traces of the lead frame are closely spaced. A center-to-center distance, or "pitch," between conductive traces may be 4 mils. Fineline tape automated bonding frames increase the chances of electrical short circuits and of electrical failure resulting from misalignment.
U.S. Pat. No. 4,696,526 to Newton et al. teaches use of alignment posts which engage holes in the tape automated bonding frame to insure proper registration of a tape automated bonded integrated circuit chip to a base unit. A method of providing precision mounted posts and precision drilled holes has worked well in most applications of aligning tape automated bonding frames. However, with fine line frames reaching pitches as small as 4 mils, it becomes more difficult to achieve the necessary accuracy by means of such a method. An accuracy of 0.3 mil is necessary to achieve an 80% overlap of outer leads of the lead frame to the connection sites of the substrate. Moreover, the method of precision mounting studs engaging precision drilled holes is an expensive one. Common materials used in the art of tape automated bonding are alumina and aluminum nitride. The placement of alignment posts through these materials is not easily accomplished and can result in a low manufacturing yield. Another problem is that the placement of posts and hole create stress points to weaken the overall structure.
Another method of aligning outer leads to a substrate is described in U.S. Pat. No. 4,899,207 to Hallowell et al. A support ring is employed to secure the outer leads in proper position for alignment with the connection sites of the substrate. The support ring prevents x-y sliding of the outer leads during bonding and prevents curling of the leads. However, the support ring is used only after the outer leads have been properly aligned with the connection sites of the substrate. That is, the support ring is used to maintain the alignment only after the exacting alignment has been achieved.
An object of the present invention is to provide a system and method for precisely aligning the outer leads of a tape automated bonding frame to connection sites on a substrate. Another object is to provide such a system and method which are reliable and cost effective.